This invention relates to glass-melting furnaces. More particularly, the invention relates to the use of electrodes inserted at selected locations through the batch of a vertical glass-melting furnace.
In an electric glass-melting furnace, electrodes are coupled to a source of electrical power and placed in contact with a bath of molten glass. Electrical energy flows between the electrodes and dissipates energy in the form of Joule heating in the molten glass for melting a blanket of glass-forming batch materials deposited on and floating atop the bath. Such electrodes may be inserted through openings in wall portions of the furnace as in conventional furnaces or may be directly placed in contact with the molten glass from above or through the layer of batch floating thereon as in the case of a cold crown electric melter.
A significant characteristic of a cold crown vertical furnace is its relatively great depth, e.g., 10'-15'. This depth is required in order to produce a specific convection pattern. An exemplary convection pattern comprises rapidly moving glass in the upper 2/3 of the furnace, sometimes hereinafter referred to as the active zone, and slower moving glass in the lower 1/3 of the furnace, sometimes hereinafter referred to as the quiescent zone. Such an arrangement gives the furnace the ability to produce quality glass at high melting rates, see U.S. Pat. No. 3,583,861. The present invention allows for the use of a relatively shallow furnace structure.
In conventional vertical furnaces, electrodes are located at the upper part of the walls near the batch blanket. Introduction of the power close to the wall causes the hottest spot in the furnace to be at the wall. As a result, the furnace suffers from high corrosion rates and a short life.
Another problem with conventional vertical furnaces is that the electrodes suffer from high corrosion and short life. The electrodes project horizontally through the furnace sidewall, and may consist of three rods with the lateral surface area oriented perpendicular to the path of electrical current flowing therebetween. Thus, corrosion is concentrated at the tip of the electrode.
In prior art furnaces, the depth of the furnace must be increased as one increases the diameter. This is partly the result of the electrical power being dissipated close to the walls so that the center of the furnace is much cooler and produces a strong downward convection which in turn reduces the thickness of the quiescent zone.
In general, electrodes positioned through the batch have the advantage of being radially and vertically adjustable within the batch blanket on the top surface of the furnace. This adjustability allows optimization of furnace performance for a particular output.
Batch electrodes are also more easily replaced than electrodes which extend through openings in the furnace sidewall. Consequently, the furnace is more reliable. Also, the batch electrode rod is now vertically placed within the furnace. With electrical current uniformly placed over the side of the rod, the corrosion of the electrode is minimized and electrode life increased.
Batch electrodes can be placed in a wide variety of positions. In general, these positions will coincide with the electrical phases available in a manner that symmetry of current flow from the electrodes is maintained. Symmetry of electrical placement and firing are important and have been found to favorably affect melting efficiency and enhance furnace life. For a suitable example, reference is directed to copending U.S. patent application Ser. No. 243,811 filed Mar. 16, 1981, now U.S. Pat. No. 4,366,571, issued Dec. 28, 1982, the teachings of which are considered to be incorporated herein by reference. The patent describes radial and circumferential arrangements of electrodes operated in a manner designed to achieve furnace symmetry.
In many glass-melting furnaces molybdenum (moly) is used as the preferred electrode material. However, because moly has a relatively low oxidation temperature of about 500.degree. C., complex protection devices are required to shield the electrodes from deterioration by contact with oxygen trapped in the glass-forming batch materials and/or other corrosive agents therein. Such devices include conventional water-cooled stainless steel sleeves or specially fabricated glass contact refractory sleeves which surround the electrode (see Ser. No. 243,811 referred to above or U.S. Pat. Nos. 2,978,526 and 4,224,460). These devices are expensive and somewhat short lived. For example, water cooling tends to dissipate energy intended for glass-melting purposes and has a deleterious effect on melting efficiency and glass quality. Protection devices tend to be heavy and cumbersome and are not easily adjusted or replaced, thereby diminishing their versatility. Glass quality may also be effected by contamination of the glass by materials forming the protective devices which materials eventually corrode and become mixed with the glass in the furnace.
A preferred embodiment of the present invention utilizes a relatively inexpensive and long-lived system for directly immersing moly rods into a bath of thermoplastic material. The moly rods are protected from oxidation without complicated peripheral apparatus. The system requires no cooling, and thus, energy utilization is enhanced. Further, the moly rods are supported in a relatively simple holder thereby facilitating adjustment and replacement.
In a series of related U.S. patent application Ser. Nos. 317,995 and 317,996 filed this same date and assigned to the assignee herein, other embodiments and examples are disclosed. The teachings of such applications are considered incorporated herein by reference.